This proposal is an attempt to analyze the structure and organization of the central nervous system at a cellular level. The important question asked is how are the terminals of afferent fibers distributed over cells of various nuclei in the brainstem. This study requires the cytological analysis of cell types in any region, the identification of afferent fibers according to source, and the analysis of their distribution patterns on cell surfaces. Electron microscopy, autoradiography, immunocytochemistry, and a number of light microscopical methods are used.